[FFmpeg-devel] [PATCH] mpeg2: fix block_last_index when mismatch control modifies last coeff

Michael Niedermayer michaelni
Mon Jun 21 20:07:11 CEST 2010

On Mon, Jun 21, 2010 at 08:00:33PM +0200, Michael Niedermayer wrote:
> On Mon, Jun 21, 2010 at 05:12:46PM +0100, M?ns Rullg?rd wrote:
> > Michael Niedermayer <michaelni at gmx.at> writes:
> > 
> > >> Having an incorrect value in block_last_index just seems wrong to me.
> > >
> > > i dont think a completely correct value would be very efficient speedwise
> > > as 75% of the cases where the last coeff is set have no effect on the post
> > > dct output for dc blocks (that is with an assumtation of dc values all being
> > > equally likely)
> > 
> > The last coeff is always manipulated, not only for dc-only blocks.  Do
> > you know for sure exactly in which cases it makes a difference?
> I dont think anything except the idct is using that coeff but i 
> i cannot formally proof that it is so or is bug free ...
> If you have doubts, its surely possible to perform some tests to see
> if its value makes a difference outside the idcts

Also to everyone to whom things feel semantically wrong
one can think of mismatch control to be part of the idct, which isnt
far fetched semantically. In that case it makes sense not to update
block_last_index. And we then would just be messing with he 64th coeff
in the coeff decoder because thats faster there than seperately later.


Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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